1. Field of the Invention
The present invention relates to an improvement in apparatus utilizing electrodialysis technology to remove unwanted by-products and to regenerate consumed sodium hydroxide in a working or operating electroless copper plating bath.
2. Description of the Prior Art
An electroless copper plating solution contains copper, usually in the form of copper sulfate, a reducing agent such as formaldehyde, a chelating agent, and an alkali metal hydroxide as essential components.
In the continued use of an electroless copper plating bath, the copper sulfate, formaldehyde, and sodium hydroxide are consumed. Depletion of these components create a need for replenishing them. Additionally, as the bath is used, by-product components are produced that have an inhibiting effect upon the chemical plating action and accumulate in the plating solution. Most notably, these are sodium sulfate and sodium formate. The build up in concentration of such by-products has a deleterious effect upon the performance of the electroless copper plating bath.
In the prior art, replenishment has usually been effected by adding at least two, and in some cases, three or more liquid concentrates to the plating bath. This procedure has several inherent disadvantages, as follows:
1. The addition of liquid concentrates results in an undesired "volume growth" of the plating bath solution. The excess solution must be bailed out, treated and disposed of as hazardous waste.
2. The liquid replenisher concentrates must be added in certain discrete ratios, which in many cases is somewhat critical, in order to maintain the chemical balance of the bath.
3. The user must chemically monitor and control certain reactant concentrations such as sodium hydroxide content.
In U.S. Pat. No. 4,289,597 issued on Sept. 15, 1981 to David W. Grenda, a method involving the use of electrodialysis is disclosed for overcoming the above disadvantages. That method has been likened to an "artificial kidney" for an electroless copper plating bath.
Electrodialysis is a form of dialysis in which an electric current is used to aid the separation of substances that ionize in solution by providing a driving potential to cause the transference of ions across semipermeable membranes. By the application of electrodialysis, the by-products may be removed from an operating electroless copper plating bath while replacing them with freshly generated hydroxide anions.
During the normal operation of an electroless copper plating bath, a chemical reaction, as follows, takes place: EQU CuSO.sub.4 +4NaOH+2HCHO.fwdarw.Cu.degree.+Na.sub.2 SO.sub.4 +2NaOOCH+H.sub.2 +2H.sub.2 O (1)
For every four moles of sodium hyrdoxide (NaOH) consumed, two moles of sodium formate (NaOOCH) and one mole of sodium sulfate (Na.sub.2 SO.sub.4) are produced. Consequently, for each complete replacement of all of the copper in the plating bath, termed a "cycle," a certain amount of sulfate and formate is produced in the bath.
With continued use and replenishment, the sulfate and formate concentrations increase steadily until the concentration reaches a level where the loss due to volume growth disposal and production rates are balanced. This is a so-called "steady state" condition. During the time between the preparation of a fresh bath and its steady state condition, the bath may display a gradual change in its performance characteristics. Thus, a "cycled" bath is usually always less stable against autocatalytic decomposition than a fresh bath. This is due primarily to the build up of sulfate and formate anions.
In traditional electrodialysis, very small electrical currents are used since charged ions are only being separated. The version of electrodialysis with which the present invention is concerned is significantly different since large electrical currents are needed. Most of this current is used to generate hydroxide ions and also transport them across the membranes.
In this version of the electrodialysis process, water is electrolyzed to form hydroxide anions at the cathode of the electrodyalysis cell. These anions subsequently migrate across an anion permeable membrane into an electroless copper bath solution which is contained in a compartment between two such anion permeable membranes. Sulfate and formate anions, together with some hydroxide, transfer across the second membrane into the anode compartment of the cell.
Three stoichiometric exchanges take place, as a result of this process, as follows:
1. 2OH.sup.- for 1 SO4.sup.=
2. 2OH.sup.- for 2OOCH.sup.-
3. 1OH.sup.- for 1OH.sup.-
Hence, the overall net exchange is:
4 OH.sup.- for 1 SO.sub.4 +2 OOCH.sup.-
Thus, for every mole of sulfate and two moles of formates removed, four moles of hydroxides are introduced. This is a perfect reversal of the reaction which takes place during electroless copper plating where four moles of hydroxides are consumed, producing one mole of sulfate and two moles of formates.
When, and if, there are no sulfates or formates to be removed, there is a simple exchange of one hydroxide for one hydroxide or a net change of zero. As a result, the bath cannot be over-replenished in caustic.
Electrodialysis apparatus for carrying out the method described in U.S. Pat. No. 4,289,597 is described in a TECHNICAL PAPER entitled "THE USE OF ELECTRODIALYSIS FOR THE CHEMICAL MAINTENANCE OF ELECTROLESS COPPER PLATING BATHS" by Dr. Alan A. Poskanzer, Dr. Melvin A. Lipson, and Mr. Stephen C. Davis of the Dynachem Corporation, a subsidiary of Morton Thiokol, Inc., and the assignee of the present invention. That TECHNICAL PAPER was presented at the PRINTED CIRCUIT WORLD CONVENTION held on May 22-25, 1984 in Washington, D.C.
The use of electrodialysis for chemically maintaining electroless copper plating baths, as described in the above TECHNICAL PAPER, has been shown to eliminate many of the inherent limitations of the preelectrodialysis prior art technology. There remain, however, a number of problems in the application of the electrodialysis technology to the chemical maintenance of electroless copper plating baths. These problems include those listed below:
1. The electrodialysis cell has a tendency to generate excessive heat, causing the temperature of the electroless copper plating bath to rise beyond control, and limiting, also, the plating bath regenerating capacity of the cell.
2. There are "dead spots" inside the electrodialysis cell where the bath tends to stagnate. In these areas, the solution has a tendency to lose stability and plate out in the cell.
3. The fluid flow is not linear in the several compartments of the cell, being very low in the catholyte and anolyte compartments (circulation in each case being dependent upon a "gas lift" principle with hydrogen being evolved at the cathode and oxygen at the anode) and very high in the regenerating or electroless copper solution compartment. This causes differential fluid pressure imbalances which tend to stretch and distend the anionic membranes and thereby cause them to rupture and cause leaking of solutions from one compartment to another, ultimately resulting in disruption of the electrodialysis process.
4. The mechanical construction of the cell is such that changing membranes is an exceedingly difficult task. The manner in which the electrodes are fabricated inside the cells tends to cause depressions and ruptures in the membranes which subsequently cause leaking of solutions.
5. The caustic generating capacity and the general efficiency of the electrodialysis apparatus is much lower than desirable.
Thus, there is a need and a demand for an improved apparatus utilizing the electrodialysis technology to remove, on a continuing basis, unwanted by-products and to regenerate sodium hydroxide in an operating electroless copper plating bath.